In general, since nitrogen oxide, sulfur oxide, soot dust, heavy metals and the like are contained in an exhaust gas discharged from a coal-fired boiler or the like, a smoke exhaust treatment device is disposed on a downstream side of the coal-fired boiler or the like. After removing a toxic substance in the exhaust gas, the thus obtained clean gas is released into the atmosphere.
FIG. 4 is an explanatory drawing showing an example of a conventional smoke exhaust treatment system. A combustion exhaust gas discharged from a boiler 1 is heat-exchanged by an air preheater 3 after removing nitrogen oxide by a denitration device 2, and then cooled to, for example, 120° C. to 170° C. Heat of the exhaust gas passed through the air preheater 3 is taken by a heating medium in a heat recovery device 4 and, after cooled to, for example, 75° C. to 110° C., smoke dust in the exhaust gas are removed by a precipitator 5. The pressure is further increased by an induced draft fan 6, and then sulfur oxide is removed by a wet-type desulfurization device 7. The temperature of the exhaust gas passed through the wet-type smoke exhaust desulfurization device usually decreases to about 40° C. to 60° C., resulting in a moisture saturation state. When the exhaust gas is released into the atmosphere as it is through a funnel to generate a white smoke, and thus the exhaust gas is heated to a dew point or higher by a reheater 18 and then discharged through the funnel 11 via a desulfurization fan 10. In this case, a heating medium circulation line 12 using a heat transfer tube is provided between the heat recovery device 4 and the reheater 18, and thus a heating medium is circulated between the heat recovery device 4 and the reheater 18 by a heating medium circulation pump 13 via the heat transfer tube. According to this system, the temperature of the exhaust gas is decreased by the heat recovery device 4 thereby allowing ash in the exhaust gas to adsorb SO3 and heavy metals, and thus enabling the removal of them together with ash by the precipitator 5. It is also possible to use heat recovered by the heat recovery device 4 for reheating of a moisture saturated gas of an outlet of the wet-type desulfurization device 7, for the purpose of preventing the generation of a white smoke through the funnel 11 (Patent Document 1).
For the purpose of reducing emissions of CO2, it has recently been planned to provide a smoke exhaust treatment system with a CO2 recovery equipment, and progress has been made in the research and development. There has been a proposition, as one of the CO2 recovery equipment, of a CO2 chemical absorption equipment in which CO2 is recovered using an aqueous solution of an amine compound such as alkanolamine and the like as an absorbing solution (for example, Patent Document 2). FIG. 5 is an explanatory drawing showing an example of a conventional CO2 chemical absorption equipment by an amine absorbing solution. The exhaust gas having a pressure raised by a blower 8 is introduced into the lower portion of an absorption column 25. After removing CO2 by contact with the amine absorbing solution fed through the upper absorption column amine feed piping 41 in an absorption column packed bed 40, the exhaust gas is washed with wash water fed through a wash water circulation line 45 in an absorption column water washing portion 42 and, at the same time, mist of the absorbing solution accompanied with the gas is removed and the gas is discharged outside as a CO2-removed gas 27. Wash water, that has flown down in the absorption column water washing portion 42, is extracted outside of the absorption column by an absorption column water washing pump 43, passed through a cooler 44 and then circulated to the absorption column through the wash water circulation line 45. On the other hand, the amine absorbing solution containing CO2 absorbed therein is stored in the bottom portion of the absorption column 25, introduced into an amine heat exchanger 46 by an absorbing solution circulation pump 28a, heated, for example, from 40° C. to 100° C., and then introduced into a regeneration column 26 through a regeneration column amine feed piping 47. In the regeneration column 26, the CO2-rich amine absorbing solution fed through the regeneration column amine feed piping 47 is fed to a regeneration column packed bed 48. On the other hand, vapor is fed to the lower portion of the packed bed 48 from a reboiler 30 through a regeneration column vapor feed piping 33. In the regeneration column packed bed 48, CO2 is eliminated in a gas phase by vapor-liquid contact between the CO2-rich amine absorbing solution and vapor. Mist of the amine absorbing solution accompanied with the eliminated CO2 gas is removed in an upper regeneration column water washing portion 49. A regeneration column outlet gas 51 is cooled to 40° C. by a regeneration column cooler 52 and condensed water drains are separated by a drum 53, followed by feeding to the regeneration column 26 as washing water 50 of the regeneration column water washing portion by means of a regeneration column wash water pump 54. In contrast, a CO2-lean absorbing solution, from which CO2 has been eliminated, is once stored in a tray 55 of the regeneration column lower portion, and then fed into a reboiler 30 from the lower portion of the reboiler through a reboiler solution feed piping 35. The reboiler 30 is provided with a heat transfer tube (not shown), and the absorbing solution is heated to, for example, 120° C. to 140° C. by passing of a steam 32 through the heat transfer tube. The heated absorbing solution generates vapor and the vapor is fed to the regeneration column 26 through the regeneration column vapor piping 33. The reboiler 30 is provided with a partition plate 56 and a solution, that has overflown the partition plate 56, is stored in the bottom portion of the regeneration column 26 through a regeneration column solution feed piping 57 and extracted by an amine absorbing solution circulation pump 28b. After decreasing the temperature to, for example, about 40° C. by the amine heat exchanger 46, the solution is fed to the absorption column packed bed 40 through the absorption column amine feed piping 41.
It is concerned that the above-mentioned CO2 chemical absorption method by an amine absorbing solution requires enormous heat, like steam, and thus drastically increases running cost. Therefore, there is proposed a method in which an amine absorbing solution fed to an absorption column from a regeneration column is preheated by using, as a unit configured to decrease heat required for a CO2 chemical absorption equipment, heat recovered by a boiler or a heating medium (for example, Patent Documents 3 and 4).